Cobalt-base alloy



- United States 2,971,224 COBALT-BASE ALLOY.

William H. Faulkner, Kokomo, Ind., assignor to Union Carbide Corporation, a corporation of New York NnDrarringt E ledDec-4, 1958, er- No. 778,053 scams. (Cl. 75-171 base alloy suitable for high temperature. servicein either.

cast. orwroughtform.

Research directed to increasingthe-power and-efiiciency ofljet propulsion and gas turbine engines have provided designs for engines that will operate athigher temperatunes than currently known metals and alloys canwithstand.v Metalsand alloys capable of withstanding these high-temperature stresses are avidly sought therefore. Furthermore, alloys are sought which, in addition to. possessing. outstandingv high-temperature strength, must also be capable of being ;fabricated into useful parts by known methods.

In some cases the alloysmay be employed in the form of castings, while forothe'r operations these alloys must becapable. ofv beinghot-worked into sheet and strip, or subjected to cold-working or machining. Furthermore, materials fabricated from-these alloys must have good oxidation, impact and thermal shock resistance.

While severalalloyshavebeen proposed forhigh:

temperatureser-vice under-the conditions setforth above, none have possessed thez required combination of--high strength, high. heat resistance and high workability;

The primary. object. of. this: invention, therefore, 7 is to. provide an alloy which has higher tensile, stressnupture, and impact.resistancethanalloys heretofore known, as well as Iincreasedductility andcreep-rupture strength, to enable its usein. awide varietyof applications, at tempcratures higher than the limitin'gvtemperatures of. cur.- rentlyknown. alloys and upto a temperature of. about 1800" Another object of thisrinventionis to provide. anallo yv withhighoxidation resistance and-fatiguestrength. at elevated temperatures,

A further objectofthe invention is to provide an alloy suitable for use at elevated-temperatures which may be fabricated by casting-.oritorging, andwwhich can be coldworked to increase, itsghardness.

Other. aims and advantages of the invention will be apparent. from the following" description. and appended claims;

In accordance with the present invention, a cobalt-base atent .In a preferred embodiment of this invention, the alloy consists essentially of about 20 percent by weight chromium, about 12.5 percent by weight tungsten, about 0.15; percent by weight carbon, up; to about 0.03 percent weight boron, up toabout 0.03-percent by weight nitro? gen, and the balance, being substantially all cobaltL'P. Chromium and tungsten both improve the high tern? perature strength of the alloy, and give the alloy increased; ability. to resist deformation while under severe; loading. Tungsten. maybe complemented by molybe denum iniamo'unts up to 3 percent by weight molybdenum. The addition ofimolybdenum will increase high temperature tensileand s'tress rupture strength at the x ense or a loss (ifrooni temperature ductility. There fore, the molybdenum content is: limitedto 3 percent. 7

While the presence of nickel increases toughness at lower temperatures it causes a reduction in high term perature strength. Tlierefore the nickel content should be.adjusted toaehievespecific desired results. For'high temperature. serviee. the. nickel. content should not be. greater. than onepercent.

Carbon andboronare.necessary additions for ductility and strength. in; products composed of this alloy. though the addition of boron greatly increases. the strength" of this alloy, boron isdetrim'entah to oxidation resistance, and therefore the. desired level. of. boron. cone: tent is. dependent. upon. the anticipated. service tempera ture oi the alloy; The boron contentis of cr-itical pbrtane'e and niust carefully controlled as indicated low. However, while higher carbon contents-are suit able for casting operations, when the ailoy i's, to-be' rolled both the carbon and boron contents should Be lowered.

The high"- temperaturestrengthof the cobalbm'atrix is furtherincreased-by the existence of a precipitated-phase; composed of carbidesofi the M G type, where M refers to the carbide formers cobalt, chromium, tungsten and molybdenum. Of especial importance are the rates: of difiusionot chromium and carbon in the, cobal'tdi'cli' matrix. Ifthese elements are able to diffuse rapidly through the matrix, the carbides are readily formed and. grow; rapidly yielding. a hardening through prec'ipitation, However, beyond a limitingl size and. con; centration,. these. carbidesare no longer useful. in increas ing the st-rengthof. the alloy. Therefore bylirnitingthe diiiusionq of; carbon and: chromium, the rate-oh growth; 01% theserjearbides-;is;diminished' andstheir; size-keptwwithi'n: limits with the: desired result that. the? alloy. achieves maximum strength fromgthisfprecipitate hardening phase.

alloy is. provided having compositions defined by: the f ranges given in .TableA..

Tungsten chromiurn, and. nickel are effective, in contre ie lt ie. f e" f. a d h 'g 't n f rbides Manganese.andsiliconmay be present ineamounts up to" 1 percent" by'weight of" each: These" elements'are' found as impurities initlie constituentspf'the alloyand; are usefulas dcoxidizersi'i f I; vIron may be presentin amounts u to 31' pereentg' u'ti since. iron'does not contribute .to high temperature strength, it should be limited 1 to amounts not.v e xc :e eding one'percentinth'e preferred alloy composition;

The amountof nitrogen in the alloy'is of. criticalim: portance; Heretofore itlwas thought--that nitrogen-iwasessential. in alloys of type, but it has been to have a deleterious elie'cton cobalt bas'ejallb B lca l fl t disete e h e e wfi, r e i basealloy otithis type.

3 TABLE B Stress-rupture data at 1700 F and 20,000 p.s.i.

Nitrogen content, percent by weight Lite, Hours ggl jgg 5 94.7 is 30.5 is 21.7 14 22.0 5 18.4 17 1 Therefore the nitrogen content should be kept below 0.06 percent by weight, and preferably below 0.04 percent for maximum strength in the alloy.

The alloy may be melted by standard furnacing proce dures, such as by induction heating, and should be conducted in the protective atmosphere of an inert gas, such as argon. This shielding of the molten metal by an inert gas will prevent contamination by the nitrogen in the air. The alloy may be used in the as-cast condition.

This alloy may also be fabricated by hot or cold working into useful parts. When ingots are required for forging, the melting, is preferably conducted under vacuum conditions. By maintaining the carbon content below about 0.20 percent by weight and the boron content below about 0.15 percent by weight, the alloy can be mechanically fabricated by known methods to produce such items as wire, strip, rod, plate, bar, sheet, tubings, forgings, and extruded shapes. These products all have excellent mechanical properties at elevated temperatures. The alloy may also be rolled into sheets. Table C gives typical stress-rupture data for sheet products having the following composition by weight: 20 percent chromium, 12.5 percent tungsten, 0.15 percent carbon, 0.028 percent boron, 0.01 percent nitrogen, and the balance substantially all cobalt.

TABLE C Stress-rupture data for 0.109 inch thick sheet Stress, Elongation, Temperature, F. p.s.i. Life, Hours Percent In another example of the invention, an alloy having a composition of about 19.98 percent chromium, about 12.35 percent tungsten, about 0.06 percent carbon, about 0.005 percent boron, about 0.48 percent silicon, about 0.48 percent manganese, and the balance substantially all cobalt was fabricated into sheet products which had the stress-rupture strengths shown in Table D.

working, the hardness of the alloy increases considerably as seen in Table E. Additionally the hardness may be further increased by subjecting the cold-worked alloy to heat-treatments. The increase in hardness may be obtained by heat-treating at temperatures as'low as 1200' F. The use of a comparatively low temperature agehardening heat treatment is'particularly significant for the low temperatures guarantee a minimum of distortion in finished parts.

TABLE E Hardness of sheet products Hardness Hardness Cold as Cold- Atter Gage, Inch Reduction, Rolled, Aging,

Percent Rockwell Rockwell 0 Test 0 Test Because of its unique properties in the cold-worked form, the alloy has a special usefulness in applications requiring high hardness for wearand abrasion-resistance;

The alloys of this invention also show a remarkable improvement in tensile properties while still possessingsuflicient ductility for a wide variety of uses. Table F demonstrates the outstanding tensile strength-ductility relationship of any alloy having the same composition as that used in the tests of Table D. 1

TABLE F Tensile test data for 0.040 inch thick sheet Yield Ultimate Strength, Elongation,- Temperature, F. Strength, 0.2% Oiiset, Percent p.s.i. p.s.l.

Room 164,000 82, 500 37 1,200 119, 500 53, 800 20 1,400 52,300 24 1,600 53, 300 39, 800 45 1 Bar stock.

High impact strength is required in many applications. Table G shows the outstanding results of impact strength tests conducted on alloy specimens having the following composition: 19.98 percent chromium, 12.35 percent tungsten, 0.16 percent carbon, 0.005 percent boron, 0.48

' percent silicon, 0.72 percent manganese, and the balance substantially all cobalt. The alloy was tested in the annealed condition with a 30 minute heat treatment at 2125 F. followed by a rapid air cool.

TABLE G Impact strength data 55 TABLE D Stress-rupture data for 0.040 inch thick sheet Temperature, 0 efififi i 93135 Strength, tt.-lbs.

Stress, Elongation, Temperature, F. p.s.i. Life, Hours Percent Rnnm 72 aa- 30, 000 59. e 43 1 'mn' 3 24, 000 47.3 73 1,600 as 15, 000 93. 8 77 ,7m 92 10,000 49. a 38 What is claimed is:

l. A cobalt-base alloy consisting essentially of from 19 to 22 percent by weight chromium, from 11.5 to 13.5 percent by weight tungsten, up to 3 percent by weight molybdenum, up to 3 percent by weight iron, up to 1 percent by weight nickel, up to 0.25 percent by weight carbon, up to 0.15 percent by weight boron, up to 0.06 percent by weight nitrogen, manganese and silicon in amounts up to 1 percent by weight of each and the balance substantially all cobalt. V

2. A cobalt-base alloy consisting essentially of about 20 percent by weight chromium, about 12.5 percent by weight tungsten, about 0.15 percent by weight carbon, up to about 0.03 percent by weight boron, up to about 0.04

percent by weight nitrogen, less than 1 percent by weight 20 percent by weight chromium, about 12.35 percent by about 0.005 percent by weight boron, about 0.48 percent by weight silicon, about 0.72 percent by weight manganese, and the balance substantially all cobalt.

References Cited in the file of this patent Woldman et 211.: Engineering Alloys, published by 

1. A COBALT-BASE ALLOY CONSISTING ESSENTIALLY OF FROM 19 TO 22 PERCENT BY WEIGHT CHROMIUM, FROM 11.5 TO 13.5 PERCENT BY WEIGHT TUNGSTEN, UP TO 3 PERCENT BY WEIGHT MOLYBDENUM, UP TO 3 PERCENT BY WEIGHT IRON, UP TO 1 PERCENT BY WEIGHT NICKEL, UP TO 0.25 PERCENT BY WEIGHT CARBON, UP TO 0.15 PERCENT BY WEIGHT BORON, UP TO 0.06 PERCENT BY WEIGHT NITROGEN, MANGANESE AND SILICON IN AMOUNTS UP TO 1 PERCENT BY WEIGHT OF EACH, AND THE BALANCE SUBSTANTIALLY ALL COBALT. 